1. Field of the Invention
The invention relates to methods for fabricating layered superlattice materials, and in particular a method that produces high quality thin films suitable for forming a portion of an electrical component in an integrated circuit.
2. Statement of the Problem
For about thirty-five years, a class of materials that spontaneously form into layered structures has been known. See Chapter 15 of the book, Ferroelectrics and Related Materials, ISSN 0275-9608, (V.3 of the series Ferroelectrics and Related Phenomena, 1984) edited by G. A. Smolenskii, especially sections 15.3-15.7; G. A. Smolenskii, A. I. Agranovskaya, "Dielectric Polarization of a Number of Complex Compounds", Fizika Tverdogo Tela, V. 1, No. 10, pp. 1562-1572 (October 1959); G. A. Smolenskii, A. I. Agranovskaya, V. A. Isupov, "New Ferroelectrics of Complex Composition", Soviet Physics--Technical Physics, 907-908 (1959); G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaya, "Ferroelectrics of the Oxygen-Octahedral Type With Layered Structure", Soviet Physics--Solid State, V. 3, No. 3, pp. 651-655 (September 1961); E. C. Subbarao, "Ferroelectricity in Mixed Bismuth Oxides With Layer-Type Structure", J. Chem. Physics, V. 34, 695 (1961); E. C. Subbarao, "A Family of Ferroelectric Bismuth Compounds", J. Phys. Chem. Solids, V. 23, pp. 665-676 (1962). Herein, we refer to the above materials, as well any other materials that can be described by a single chemical formula and which spontaneously form themselves into alternating layers having distinctly different crystalline structure, as layered superlattice materials. The term "layered superlattice material" is selected to distinguish these superlattice materials from alloy type superlattice materials, which are not layered, and superlattice heterostructures, which are inherently not a "material" but rather layered structures made of at least two different materials having different chemical formulae.
Until now, the only known generalized method of making layered superlattice materials as been powder metallurgy. Powder metallurgy methods of producing films involves grinding and mixing of powders, forming a slurry with water or some other carrier, applying the slurry to substrates by spraying, painting or the like, and then firing the applied films at high temperatures such that the carrier is driven off and the particles are melted together to form the desired film. It is impossible to make thin films suitable for use in integrated circuits with this method. It has been attempted to make a few two-metal layered superlattice materials, such as bismuth titanate and magnesium fluoride, using conventional vacuum sputtering, chemical vapor deposition, growing single crystals and sol-gel methods. See for example, S. Y. Wu, "Memory Retention and Switching Behavior of Metal-Ferroelectri-Semiconductor Transistors", Ferroelectrics, Vol. 11, pp. 379-383, 1976, U.S. Pat. No. 4,792,463 issued Dec. 20, 1988 to Masaru Okada et al., European Patent Application publication No. 0415 751 A1, published Mar. 6, 1991, and U.S. Pat. No. 5,146,299 issued Sep. 8, 1992 to Donald R. Lampe et al. The Lampe et al. patent also mentions that a sol-gel process may be used to make some materials, which we have called layered superlattice materials (but Lampe et al did not recognize as such), but does not give any examples, and states that the suggestion is based on the demononstration of the sol-gel process to be a viable technique for making high purity, stoicheometric oxide materials, which was work performed by the present inventors and others. See, for example, B. M. Melnick, J. D. Cuchiaro, L. D. McMillan, C. A. Paz De Araujo, and J. F. Scott, "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990). However, conventional sol-gel processing results in materials that crack unpredictably, are too thick, which fatigue, and generally are of insufficient quality for use in integrated circuits. See, "Synthesis of Metallo-organic Compounds for MOD Powers and Films", G. M. Vest and S. Singaram, Materials Research Society Symposium Proceedings, Vol. 60,1986 pp. 35-42 and J. V. Mantese, A. L. Micheli, A. H. Hamdi, and R. W. Vest, "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", MRS BULLETIN, October 1989, and the Melnick article referenced above.
None of these isolated attempts to make materials of suitable quality for a memory indicated that the methods could be used for layered superlattice materials in general, and in fact none even recognized this class of materials. Moreover, none of these methods proved effective enough to actually be applied to making an integrated circuit. Thus, up to now, there has been no effective method for making thin films of layered superlattice materials that are of suitable quality for use in integrated circuits.
The layered superlattice materials having three or more metals, such as strontium bismuth tantalate or strontium bismuth tantalum niobate, are much harder to make than the two-metal layered superlattice materials, such as BaMF.sub.4. For example, while single crystals of BaMF.sub.4 and other two-metal layered superlattice materials have been grown in many laboratories (see the Lampe et al. patent cited above), no one has yet reported fabrication of a single crystal of any layered superlattice material having three or more metals. Similarly, while several papers have reported making certain two-metal layered superlattice materials by sputtering, no one has reported the fabrication of any layered superlattice materials having three or more metals by sputtering. Since, as will be seen below, layered superlattice materials having three or more metals are much more useful than the two-metal materials, there is an specially significant need for a generalized process by which high-quality layered superlattice materials having three or more metals can be made.